This invention relates to an organic compound additive for liquid hydrocarbon fuels and liquid hydrocarbon fuel compositions containing such additive. The additive is useful for improving the ignition quality and increasing the cetane number of liquid hydrocarbon fuels, and more particularly relates to diesel fuels with enhanced cetane numbers.
Diesel engines use the heat developed by compressing a charge of air to ignite the fuel injected into the engine cylinder after the air is compressed. More specifically, in the engine, the air is first compressed, then fuel is injected into the cylinder; as fuel contacts the heated air, it vaporizes and finally begins to burn as the self-ignition temperature is reached. Additional fuel is injected during the compression stroke and this fuel burns almost instantaneously, once the initial flame has been established.
A period of time elapses between the beginning of fuel injection and the appearance of a flame in the cylinder. This period is commonly called "ignition delay," and is a major factor in regard to the performance of a diesel fuel. If ignition delay is too long, the fuel will accumulate in the cylinder until it reaches ignition conditions and then will burn rapidly, causing a sudden pressure increase which may result in engine knocking. Too long an ignition delay may result in a smokey exhaust, a decrease in engine efficiency, and possibly dilution of the crank case oil. A reduction in ignition delay can be obtained by varying the chemical nature of the injected fuel. Straight-chain paraffinic hydrocarbons give the least ignition delay, while branched-chain paraffins and cyclic (including aromatic) hydrocarbons tend to have poorer ignition characteristics. For this reason, n-hexadecane ("cetane"), which is a diesel fuel having excellent ignition qualities, has long been used as a standard reference material for determining the ignition quality of commercial diesel fuels. A scale called "cetane number" has been devised for ranking the relative ignition delay characteristics of a given diesel fuel. The cetane number of an unknown fuel is determined by comparing its ignition delay in a standard test engine with reference fuels which are prepared by blending cetane (assigned a rating of 100) and 2,2,4,4,6,8,8-heptamethylnonane (assigned a rating of 15) until a reference fuel is found to have the same ignition delay characteristics as the unknown fuel; the cetane number is obtained by the equation: EQU Cetane No.=(Vol. % Cetane)+[0.15 (Vol. % Heptamethylnonane)]
In general, large stationary engines which run at fairly constant speeds and loads have the lowest cetane number requirements (e.g., 30 to 45), while smaller, motor vehicle diesel engines have the highest requirements (e.g., 40 to 55) for obtaining optimum performance.
One of the more important difficulties that arises through the use of diesel engines is the problem of starting the engine when it is cold. Fuels with high cetane numbers have the advantage of giving relatively easy starting at low temperatures. Additionally, fuels with high cetane numbers reduce destructive combustion knock, provide more efficient combustion and smooth engine operation, lower maximum cylinder pressures, and reduce carbon deposits on cylinder heads.
Historically, diesel engines have been operated on a petroleum-derived liquid hydrocarbon fuel boiling in the range of about 300.degree.-750.degree. F. (149.degree.-399.degree. C.) Modern petroleum refineries can produce high quality diesel fuels containing large straight-chain paraffins. However, due to competing demands for other products, limitations imposed by poor quality heavy crude oils, and other factors, refineries frequently are unable to meet the total demand for such diesel fuels. Because of these refining constraints, and dwindling petroleum reserves, alcohol-hydrocarbon blends have been studied for use as diesel fuels. The addition of alcohol to the hydrocarbon liquid tends to produce a fuel with a further reduced cetane number.
Through the years, many types of additives have been prepared to raise the cetane number of diesel fuels. Such additives usually contain nitrogen or sulfur, both of which are known cetane improvers under certain circumstances. The most popular additives, for example, appear to be hexyl or octyl nitrate. However, these additives are highly combustible, as are most of the organic nitrogen- or sulfur-containing additives commonly used. Further, the nitrogen-containing compounds can add to an engine's NO.sub.x emissions, which contribute to photochemical reactions known to cause smog formation, as well as formation of nitric acid, a factor in acid rain. The sulfur-containing compounds contribute to SO.sub.x formation which can contaminate the lube oil by forming sulfuric acid which breaks down various antiwear additives found in the oil. Also, SO.sub.x emissions contribute to the formation of particulate matter in the form of sulfates which must be emitted from the exhaust.
With the continued increase in demand for diesel fuel in both passenger cars and trucks and the economic restraints on the quality of available diesel fuel, there is a need for new, safer, and more economical cetane boosters. Accordingly, it is a feature of the present invention to provide new, safer, and more economical cetane boosters for middle distillate fuels, more particularly diesel fuel.